The present invention, relates to navigation satellite receivers, and more particularly to methods and systems for operating navigation satellite receivers in faint signal-level environments.
The global positioning system (GPS) is a satellite-based radio-navigation system built and operated by the United States Department of Defense at a cost of over $13 billion. Satellite positioning systems (SPS) include GPS and the Russian GLONASS navigation system. Others are proposed by Japan and the European Union.
In the GPS system, twenty-four satellites circling the earth at an attitude of 20,200 km are spaced in orbit such that a minimum of six satellites are in view at any one time to a user. Each such satellite transmits an accurate time and position signal. GPS receivers measure the time delay for the signal to reach it, and the apparent receiver-satellite distance is calculated from that. Measurements like this from at least four satellites allow a GPS receiver to calculate its three-dimensional position, velocity, and system time.
A navigation satellite receiver that has just been turned on does not yet know the exact orbital positions of satellites that are visible to it, where the receiver itself is, how much its crystal oscillator is in error and therefore its tuning frequencies, nor what time it is. It may however, know time to within a few seconds or better, and its rough position to within a hundred kilometers. Such prior knowledge, even so rough as this, can be used to great advantage.
The exact system time and the apparent carrier frequencies from the satellites are needed by the receiver to find and lock onto the transmissions, and so initially a search is usually needed of all the possibilities. Reducing the range of possibilities leads directly to quicker initializations. It is a commercial necessity for manufacturers of such equipment that the first position fix be well within the limited patience of the typical user.
A GPS receiver that is associated with a cellphone or that can communicate over the Internet can be assisted in many ways by network servers connected to other GPS receivers that already have satellite-lock and are tracking. A telephone or network communication channel can be used to contribute key bits of information to a navigation satellite receiver to help it initialize faster. One of the present inventors, Paul McBurney, and others, have recently filed several U.S. patent applications that relate to aiding GPS receiver clients. These are summarized in Table I, and all such patent applications have been assigned to the same Assignee, and are incorporated herein by reference.
The GPS satellites transmit a 50-bps navigation (NAV) data message that repeats every 12.5 minutes. It comprises system time, satellite ephemeris, and almanac information that is critical to a GPS receiver in acquiring signal lock on enough satellites and producing its navigation solutions. There are twenty-five frames that each take 30-seconds, each frame has five subframes, and each subframe has ten words. A Z-count at the beginning of each subframe gives its transmission time from the satellite. Ephemeris is the first three subframes, and subframes 4-5 are almanac data spread over fifty pages. One whole data frame of NAV data is 1500-bits long, and thus takes thirty seconds to transmit.
The NAV-data cannot be reliably received and demodulated if its signal level is too weak. Such can occur indoors or below decks. So high sensitivity receivers need informational assistance from a third party over a different channel to get the current NAV-data. If local-receiver system-time is known, the z-count information can be plugged into an otherwise generic NAV-data message obtained from such third party.
Each data frame is divided into five subframes 1-5, and each subframe is 300-bits long, e.g., ten 30-bit words. Thus it takes six seconds to transmit each 300-bit, 10-word subframe. Every subframe starts with a telemetry (TLM) word of 30-bits, followed by a hand-over word (HOW) of 30-bits. Both 30-bit words comprise 24-bits of data and 6-bits of parity. There are eight words of data payload in each subframe.
The TLM word at the front of each 300-bit subframe begins with an 8-bit preamble. The preamble allows the start of a subframe to be recognized, and thereafter provides a primary mechanism for the receiver to be synchronized.
The first 300-bit subframe transmits the satellite vehicle (SV) clock correction data after the TLM word and HOW. The second subframe transmits the first part of the satellite vehicle-ephemeris data. The third subframe transmits the second part of the satellite vehicle-ephemeris data. Subframes four and five are used to transmit different pages of system data. The fourth subframe also begins with the TLM word and HOW, and the data payloads rotate over 12.5 minutes to transmit the lengthy information about the ionosphere, UTC, and other data. An entire set of twenty-five frames, e.g., 125 subframes, makes up the complete Navigation Message that is sent over such 12.5 minute period. The fifth subframe begins with the TLM word and HOW, and its data payload also rotates over 12.5 minutes to transmit the rather-large almanac.
The clock data parameters describe the satellite vehicle-clock and its relationship to GPS time. The ephemeris data parameters describe satellite vehicle-orbits for short sections of the satellite orbits. Normally, a receiver gathers new ephemeris data each hour, but it can use old data for up to four hours without much error. The ephemeris parameters are used with an algorithm that computes the satellite vehicle position for any time within the period of the orbit described by the ephemeris parameter set. The almanacs are approximate orbital data parameters for all satellite vehicle""s. The ten-parameter almanacs describe satellite vehicle orbits over extended periods of time, and is sometimes useful for months.
The signal-acquisition time of a GPS receiver at start-up can be significantly speeded by having the current almanac available. The approximate orbital data is used to preset the receiver with the approximate position and carrier Doppler frequency of each satellite vehicle in the constellation.
Norman F. Krasner describes a way to deal with NAV-data messages that cannot be read because the carrier signal levels are too weak, in METHOD AND APPARATUS FOR SATELLITE POSITIONING SYSTEM BASED ON TIME MEASUREMENT, U.S. Pat. No. 6,239,742 B1, issued May 29, 2001. A base station is used to record parts of the NAV-data message and these are compared to similar data from a remote SPS receiver. The remote SPS receiver receives parts of the NAV-data message directly from satellites visible to it. The NAV-data recorded by the base station includes with it correct time identification, so matching up the two overlapping-in-time parts can assist the remote SPS receiver in finding its correct system time. Such comparison is not done at the remote mobile receiver, but rather back at the base station.
It is therefore an object of the present invention to provide a method and system for assisting navigation satellite reception and receiver initialization.
It is another object of the present invention to provide a method and system for reducing the time necessary for GPS and SPS receivers to initialize.
It is a further object of the present invention to provide a satellite-navigation system that is cost effective.
Briefly, a navigation satellite receiver method embodiment of the present invention determines what navData is on-hand, what level of time uncertainty exists, and what position uncertainty there is for the receiver at turn-on. Indoor and outdoor search engines are used that can vary their search windows and dwell times to increase receiver sensitivity. Received signals are stored in several playback loops that can be operated in parallel to increase search sensitivity in the face of large uncertainties in time and frequency, and still reduce the time-to-first-fix. Satellite acquisition can be achieved even when the navData is too weak to be read by requesting help from a server.
An advantage of the present invention is that a system and method is provided that provides for initialization of navigation satellite receivers in attenuated signal attenuation levels that would otherwise not be able to initialize.
Another advantage of the present invention is that a system and method is provided for reducing the cost navigation satellite receivers associated with mobile cellular telephones.